1. Field of the Invention
The present invention relates in general to powder metal engine components, and more particularly to a new and improved powder metal valve guide for high temperature applications.
2. Description of the Related Art
Valve guides are typically tubular structures constructed to receive the valve stem of an engine poppet valve in an internal combustion engine. The construction of these engine components is well known to those skilled in this art.
Powder metal (P/M) valve guides are made from relatively low alloy steels containing ferritic/pearlitic microstructures with solid lubricants such as silicates, free graphite, manganese sulfide, copper sulfide or molybdenum disulfide. The prior art P/M valve guide is pressed to a low to medium density, sintered using conventional sintering temperatures, such as less than about 1,150° C., and then machined at both ends. The inner bore is formed by reaming. It is known in the art to oil impregnate the valve guides for extending their life. The operation of the internal combustion engine replenishes the valve guides with oil. The life expectancy of the valve guides relies on the engine oil to lubricate the interface between the valve stem and the valve guide. Recently, there have been efforts to design what may be termed as “oil starved” valve guides to address the problem of air pollution caused by engine lubricant oil leaking into the combustion chamber through the valve stem and valve guide interface.
U.S. patent application Ser. No. 09/969,716, filed Oct. 2, 2001 by the Assignee of the present invention, which is hereby incorporated by reference herein, is directed to such a valve guide capable of withstanding high temperatures with little or no lubrication. The valve guide according to that invention was particularly intended for use in a cooled cylinder block of an internal combustion engine.
Other applications for a valve guide can include locations where the valve guide is exposed to high temperatures such as in excess of about 1000° F. in a system that is not cooled. For example, an exhaust gas recirculation (EGR) valve is disposed between an engine exhaust manifold and the engine intake manifold. The EGR valve uses a poppet valve (which includes a valve guide) to permit the recirculation of exhaust gas from the exhaust side of the engine back to the intake side. As is known to those skilled in the art, such recirculation of exhaust gasses is helpful in reducing various engine emissions.
It has become desirable to operate the EGR valve in a continuously variable mode responsive to control signals from the engine control unit (ECU) for optimum engine performance while simultaneously minimizing emissions. As a result, the poppet valve and valve guide in the EGR valve are exposed continuously to the high temperatures and corrosive properties of the exhaust gas for prolonged periods of time.
The excessive temperature can negatively affect the performance of the components, and particularly the performance of the reciprocal movement of the valve stem within the valve guide such as, for example, the valve sticking or seizing within the valve guide. The corrosive materials found in the exhaust stream further negatively impact the life of the components.
Thus, there still exists a need for a powder metal valve guide capable of withstanding the significantly high temperatures found in EGR valve applications as well as being useful in other high temperature applications where the valve guide is provided with little or no lubrication, or cooling.